Using Hodographs Matthew J. Bunkers NWS Rapid City, SD IAS Seminar Rapid City, SD 11/2/2010

Why should we care about hodographs? 2 Wikipedia NWS UNR

Why should we care about hodographs? 3 © Tom Warner Unknown NWS UNR

Why should we care about hodographs? 4

5 Storm type Severe threat Storm motion Infer thermal advection Infer quadrant of cyclone/anticyclone –Bluestein and Banacos (2002, MWR) Turbulence, mountain waves Others…

First, what is a hodograph? 6 Something all severe weather forecasters should know how to use and interpret A line connecting the tips of wind vectors between two arbitrary heights in the atmosphere A plot of vertical wind shear from z 1 to z 2

7 Ground-relative winds

8 There is nothing magic about 0  6 km

9 VV  V = V top – V bot (shear vector) V bot V top

10 Shear also can be displayed this way, but it doesn’t show direction

11 Bulk shear =  V between two levels 0  6-km shear vector Mag. = 35.9 m s -1 Shear = s -1 (disregards shape of the hodograph)

12 Total shear = sum of  V over layers Mag. = 51.0 m s -1 Shear = s -1 (sensitive to depth of shear layers, curvature, loops, and “wiggles”)

13 Positive shear = clockwise/neutral turning Mag. = 40.1 m s -1 Shear = s -1 (sensitive to depth of shear layers, curvature, loops, and “wiggles”)

14 0  6-km “shear” Bulk = 15.7 m s -1 Total = 38.5 m s -1 Pos. = 23.7 m s -1 (loops and wiggles are a problem here) When in doubt, look at the hodograph

15 Effective shear may be more appro- priate than 0–6-km shear at times Thompson et al. (2007, WAF)

16 Total shear always  bulk shear Bunkers et al. (2002, 21 st SLS:

Not all hodographs are created equal 17 Height intervals should be even and labeled, with shear highlighted by varying colors –E.g., plot every 0.5 km; dot or label every 1 km Some hodographs have uneven spacing Some hodographs are not labeled Some hodographs don’t vary the color –Distribution of shear is important

18 © Bunkers Also see NSHARP NWS AWIPS

Hodographs can be difficult to visualize from a sounding alone 19 Markowski and Richardson (2006, WAF)

Hodographs can be difficult to visualize from a sounding alone 20 Markowski and Richardson (2006, WAF)

21 Why is shear important? hh Markowski et al. (2008, MWR) Handout hodograph and plot  h and V SR

22 V2V2 V1V1  h = k ×  V h /  z (horizontal vorticity*) 22 * Ignoring horizontal changes in vertical wind Horizontal vorticity points to left of shear 11

23 Storm-relative vs. ground-relative winds V - C = V SR = SR wind V SR V C

24 Storm-relative vs. ground-relative winds The storm- relative winds matter most because that is what the storm “sees” (e.g., see next slide) V - C = V SR = SR wind V SR

25 Differences in storm-relative flow can alter anvil orientations Lindsey and Bunkers (2005, WAF)

26 Storm-relative helicity (V SR and  h ) SRH =  (V SR   h ) dz 11 22 33 44 55 66 V SR   h leads to pure cross- wise vorticity SRH 0  3 = 0 for storm motion along straight hodograph

27 V SR ||  h leads to pure stream- wise vorticity SRH 0  3 >> 0 for motion at center of circu- lar hodograph (621 m 2 s -2 or J kg -1 in this case) Storm-relative helicity (V SR and  h ) 11 22 33 SRH =  (V SR   h ) dz

28 Usually a mix of streamwise and crosswise vorticity is observed SRH =  (V SR   h ) dz SRH 0  3 = -2  area swept out by V SR on hodograph SRH 0  1 (or 0.5 km) is rather important for tornadoes

29 SRH often has large variability in both time and space (in the CBL) SRH 0  3 also can vary over 100 m 2 s -2 for different  z’s Markowski et al. (1998, 19 th SLS, ) Markowski and Richardson (2007, MWR)

Anticipating storm motion is very important for SRH, and other indices 30 © Mike Umscheid Ziebach County/NWS UNR © Matt Bunkers © Brian Morganti

31 Storm motion is comprised of advection and propagation Advection –Mean wind Propagation –Updraft  shear interactions (for supercells) –Gust-front propagation –Boundary layer convergence features –Storm mergers and interactions –Orographic features –Others (e.g., gravity waves)

32 Supercell motion is dominated by advection and updraft  shear forcings © COMET

33 Most of the time supercell motion can be predicted well using a hodograph 1)Plot a representative mean wind (e.g., 0- 6km, 0-8km, 1-7km) Bunkers et al. (2000, WAF); Zeitler and Bunkers (2005, NWD) 2)Draw a shear vector from the BL to 5.5-6km 3)Draw a line that both passes through the mean wind and is orthogonal to the shear vector (i.e., the updraft- shear propagation component) 4)Plot the RM and LM supercells 7-8 m/s from the mean wind (this can be variable)

Methods to predict supercell motion should be Galilean invariant 34 Supercell motion based on mean wind (e.g., 30° to the right and 70% of the mean wind speed) is not Galilean invariant 30R75

35 #1 #2 #3#4 Time for some practice…plot the supercell motion

Case #1 36

Case #2 37

Case #3 38

Case #4 39

Time for hodograph demo 40

41 Now you have to make a choice You take the blue pill, and the seminar ends… …or you take the red pill, and you learn more about hodographs “Remember that all I am offering is the truth. Nothing more” - Morpheus

The length and shape of the hodo- graph helps determine storm type 42 (or splitting supercells)

The length and shape of the hodo- graph helps determine storm type 43 (or splitting supercells) Courtesy of Paul Markowski

The length and shape of the hodo- graph helps determine storm type 44 Dominant RMsDominant LMs

Real-world observations tend to support this 45 Bunkers (2002, WAF)

The distribution of the shear helps determine severe threat 46 Severe SCs Non-svr SLs Severe SLs Severe Bows Sig TOR SCs Klimowski et al. (2003, WAF) Esterheld and Giuliano 2008, EJSSM) Rapid City flood

47 Other shear-related indices are derived from either shear or SRH BRN = CAPE / (bulk shear) Other common indices include EHI, VGP, SCP, and STP

48 SRH also can be used as a proxy for 850  700-mb temperature advection

No! 49 Can you become “friends” with hodographs on Facebook?

50 But you can find out more here… Doswell, C. A., III, 1991: A review for forecasters on the application of hodographs to forecasting severe thunderstorms. Natl. Wea. Dig., 16, 2– Hodograph spreadsheets: Principles of Convection II : Using Hodographs Predicting Supercell Motion in Operations ftp://rammftp.cira.colostate.edu/bikos/audio/scmotion_audio.exe ftp://rammftp.cira.colostate.edu/bikos/audio/scmotion_audio.exe